In wire-line communications, a transmission line carries electrical signals from a transmitter end to a receiver end. At the transmitter end, the driver circuits feed electrical signals, such as voltage signals, into the transmission line. The driver circuits are commonly known as line drivers. Ideal line drivers transfer the entire signal power generated by the transmitter to the transmission line. In practice, the highest power transfer between the line driver and the transmission line occurs when the output impedance of the line driver matches the characteristic impedance of the transmission line. Also, depending on the circumstances, the line driver may also be required to provide a certain signal voltage level at the input terminals of the transmission line.
In some versions of Ethernet communication, such as Fast Ethernet (100 BASE-T2) or gigabit Ethernet (1000BASE-T), pulse amplitude modulation (PAM) is used to encode message information in the amplitude of a series of signal pulses. Therefore, in PAM, the pulse amplitude represents the actual message data and must be preserved. Line drivers for PAM applications are typically designed for maintaining pulse amplitude variations, while being mindful of impedance matching (between the output impedance of the line driver and the characteristic impedance of the transmission line) and minimizing power consumption by the line driver circuit.
FIG. 1 illustrates a conventional line driver circuit 100 using variable resistors that form a pull-up, pull-down path. Line driver circuit 100 includes a voltage buffer 110, a floating variable resistor RM, respective first and second variable resistors RS, and four switches S1a, S1b, S2a, and S2b. The states or positions of switches S1a, S1b, S2a, and S2b and the values of variable resistor RM and first and second variable resistor RS are controlled by driver coder logic 130. Switches S1a and S1b are controlled by the output signal S1 and switches S2a and S2b are controlled by the output signal S2 of driver coder logic 130.
Although conventional line driver circuits often work well for their intended applications, the load current drawn from a power supply is often input-data dependent. The load current is generally at its highest value when an output voltage amplitude (at input terminals of transmission line 120) is at its highest value, and zero when the output voltage amplitude is zero. While this helps to reduce the average power consumption significantly if the data is not just flipping between +1 and −1 (as in a Non Return to Zero 2-level Pulse Amplitude Modulation (NRZ 2-PAM)), for the NRZ N-PAM data streams where N>2, this results in large variation of the current drawn from the power supply, which in turn translates into wide-band supply noise. Moreover, the power supply-rejection ratio (PSRR) of line driver 100 is fairly low (e.g., around 6 dB when terminated by a line having a characteristic impedance equal to the output impedance of liner driver 100). Thus, self-induced high frequency noises as well as other noises and ripples on the power supply may directly contribute to distortion of the output signal provided to transmission line 120.
For sensitive applications (e.g. 10GBase-T), where the link SNR is critical, the quality of the transmitted signal is of utmost value and typically a higher PSRR value (e.g., around 60 dB) is required of the line driver. Thus, the need exists for a line driver that can reduce the data dependency of the current drawn from the power supply, therefore alleviating the disadvantages of the conventional solutions. Embodiments described herein satisfy this need.